Recently, developments of a semiconductor laser device integrating a semiconductor laser and a modulator have been advanced, intended for an application to optical communications. In this device, a distributed feedback laser diode (DFB LD) is operated with direct current and light emitted from the laser diode is subjected to high speed modulation by a light absorption modulator.
FIGS. 24(a)-24(c) are diagrams illustrating a prior art integrated modulator and semiconductor laser device. FIG. 24(a) is a perspective view of the semiconductor laser device, FIG. 24(b) is a cross-sectional view of the device taken along a line 24b--24b, at the isolation part between the laser and the modulator, and FIG. 24(c) is a cross-sectional view of the device taken along a line 24c--24c in the resonator length direction.
In these figures, reference numeral 1 designates an n type InP substrate. A ridge, i.e., optical waveguide, comprising an active layer 2 and a p type InP cladding layer 3 is disposed on the semiconductor substrate 1. This ridge includes a semiconductor laser part (a), and a modulator part (c) having a diffraction grating (not shown) at the surface of the semiconductor substrate 1 beneath the active layer 2. Burying layers comprising an Fe-doped InP semi-insulating semiconductor layer 6a, an n type InP hole trap layer 8 and an Fe-doped InP semi-insulating semiconductor layer 6b, successively formed, are disposed on both sides of the ridge. A p type InP cladding layer 10 and a p type InGaAs contact layer 4 are successively disposed on the ridge and on the burying layers. The contact layer 4 is absent at an isolation part (b) between the laser part (a) and the modulator part (c). The entire surface of the wafer, except for portions of the contact layer 4, is covered with an insulating film 7.
A description is given of the operation.
Since this semiconductor laser device has a diffraction grating beneath the active layer 2 of the modulator part (c), laser oscillation at a single wavelength is stably performed employing the diffraction grating. The active layer 2 of the laser part (a) and the active layer, i.e., light absorption layer, 2 of the modulator part (c) comprise a continuous InGaAs/InGaAsP multiple quantum well layer. The energy difference between normal levels of the conduction band and the valence band in the quantum well of the laser part (a) is smaller than that of the modulator part (c). Therefore, when no bias voltage is applied to the modulator part (c), light from the laser part (a) is not absorbed in the light absorption layer 2. However, if reverse bias voltage is applied to the modulator part (c), the light is absorbed due to quantum confined Stark effect (QCSE). Consequently, the light emitted from the laser part (a) which is operated with direct current, can be modulated by changing the bias voltage which is applied to the modulator part (c).
In addition, the Fe-doped InP semi-insulating semiconductor layers 6a and 6b and the n type InP hole trap layer 8 fill in both sides of the ridge waveguide comprising the active layer 2, and the InP cladding layer 3 and the InP substrate 1 respectively disposed above and below the active layer 2, and function as a current blocking layer. Because Fe produces a deep acceptor in InP, the Fe-doped InP semi-insulating semiconductor layer 6a can block diffusion of electrons from the n type InP substrate 1. The n type InP hole trap layer 8 can block diffusion of holes from the p type InP cladding layer 10. Thereby, the threshold current of the laser is reduced and the efficiency of the laser is enhanced.
A description is given of the fabricating method.
FIGS. 21(a)-21(c), 22(a)-22(c) and 23(a)-23(c) are diagrams illustrating respective states of process steps in the fabricating method.
Initially, after forming the diffraction grating in a region of the n type InP semiconductor substrate 1 where the modulator is to be formed, the active layer 2 comprising an InGaAs/InGaAsP multiple quantum well layer and the cladding layer 3 comprising, for example, p type InP, are grown on the substrate 1 by MOCVD. Then, an insulating film stripe 5 having a width of 1.about.2 .mu.m is formed on the cladding layer 3. Using this stripe 5 as a mask, dry etching is performed to form a ridge having a height of about 2.about.3 .mu.m, thereby providing an optical waveguide (FIGS. 21(a)-21(c)). Thereafter, the Fe-doped InP semi-insulating semiconductor layer 6a, the n type InP hole trap layer 8 and the Fe-doped InP semi-insulating semiconductor layer 6b are successively grown on both sides of the ridge (FIGS. 22(a)-22(c)). After removal of the insulating film stripe 5, the p type InP cladding layer 10 and the p type InGaAs contact layer 4 are successively grown by MOCVD (FIGS. 23(a)-23(c)). Then, the contact layer 4 at the isolation part (b) between the semiconductor laser part (a) and the modulator part (c) is etched and removed. Further, in order to make the capacitance lower, opposite sides of the active layer 2 are etched to form a mesa shape. In this etching, the ridge width of the contact layer 4 is 10.about.20 .mu.m, and the etching range of the contact layer 4 in the resonator length direction, is 10.about.50 .mu.m. The entire surface of the wafer, except for portions of the contact layer 4, is covered with the insulating film 7, completing the semiconductor laser device shown in FIG. 24(a). In addition, an electrode for making an ohmic contact is formed on the portions of the contact layer 4 unmasked with the insulating film 7.
In the integrated modulator and semiconductor laser device, since the semiconductor laser part is forward biased and the modulator part is reversely biased, sufficient electrical isolation between the laser part and the modulator part is required. Generally, an oscillation wavelength of a semiconductor laser varies with varying injection current. In the integrated modulator and semiconductor laser device, however, the laser part is driven with a constant current. Thus, if the electrical isolation between the laser part and the modulator part is insufficient, the current flowing through the laser part is unfavorably varied by a modulation signal which is applied to the modulator part via the isolation resistance between the laser part and the modulator part, thereby resulting in wavelength variation. This wavelength variation causes degradation of a transmitted waveform during long-distance transmission through an optical fiber, resulting in limited transmission distance. Consequently, in order to improve transmission characteristics, it is very important to enhance the isolation resistance between the laser part and the modulator part.
On the other hand, the mobility of electrons is tremendously larger than that of holes in InP. This results in low electrical resistance of the n type InP hole trap layer 8 in the above-described semiconductor laser device. In addition, the hole trap layer 8 is continuous through the modulator part (c) and the laser part (a). This results insufficient electrical isolation between the modulator part (c) and the laser part (a), and there occurs mutual interference between these parts. Further, the semi-insulating semiconductor layer 6b forms a capacitance. Accordingly, when the hole trap layer 8 is continuous through the modulator and laser parts, the capacitances of the semi-insulating semiconductor layer 6b are coupled, thereby increasing parasitic capacitance of the modulator part (c) and impeding modulation operation at harmonics.
The following are semiconductor laser devices which avoid these problems.
A semiconductor laser device shown in FIG. 25 integrates a laser part (a) and a modulator part (c). The laser and modulator parts (a) and (c) are both disposed on a semiconductor substrate 100. The laser part (a) includes optical waveguide layers 15 and 16 and a quantum well layer 50a, and the modulator part (c) includes optical waveguide layers 15 and 16 and a quantum well layer 50b and has a diffraction grating 12 formed on the semiconductor substrate 100. P.sup.+ type InGaAsP layers 70 are disposed on the laser and modulator parts (a) and (c) and electrodes 91a and 91b are disposed on the p.sup.+ type InGaAsP layers 70. There is an isolation part (b) comprising a p type InP layer 65 having a larger band gap energy than those of the optical waveguide layers 15 and 16, provided between the laser part (a) and the modulator part (c). This p type InP layer 65 of the isolation part (b) increases the isolation resistance between the laser part (a) and the modulator part (c).
A semiconductor laser device shown in FIG. 26 is provided with a laser part (a) and a modulator part (c) on a semiconductor substrate 101. In the laser part (a), a first optical guide layer 102, a first buffer layer 103, a second optical guide layer 106, a cladding layer 107 and a cap layer 108 are successively disposed on the semiconductor substrate 101. The modulator part (c) has a diffraction grating 12 formed on the semiconductor substrate 101, the first optical guide layer 102 and the first buffer layer 103, which are continuous with the respective layers of the laser part (a), disposed on the semiconductor substrate 101. An active layer 104, a second optical buffer layer 105, a second optical guide layer 106, a cladding layer 107 and a cap layer 108 are successively disposed on the first buffer layer 103. There is an isolation part (b) filled with a high resistance semiconductor 109 between the laser part (a) and the modulator part (c). This high resistance semiconductor 109 increases the isolation resistance between the laser part (a) and the modulator part (c).
A semiconductor laser device shown in FIG. 27 includes a ridge, i.e., optical waveguide, 140 comprising an active layer 110, 400 and a cladding layer 350 that is disposed on a semiconductor substrate 200. The ridge 140 includes a semiconductor laser part 1010 and a modulator part 1020 having a diffraction grating 120 on the semiconductor substrate 200 beneath the active layer 110. Burying layers comprising a semi-insulating semiconductor layer 800 and a hole trap layer 900, which are successively formed, are disposed on both sides of the ridge 140. Electrodes 300 and 700 are disposed on the lower and upper surfaces of the semiconductor laser device, respectively. In this semiconductor laser device, the cladding layer 350, the hole trap layer 900 and the semi-insulating semiconductor laser 800 between the laser part 1010 and the modulator part 1020 are etched and removed, thereby increasing the isolation resistance between the laser part 1010 and the modulator part 1020.
In the semiconductor laser devices shown in FIGS. 25 and 26, however, the active layers in the laser part (a) and the modulator part (c) are separated from each other. Therefore, the optical coupling efficiency between the laser part (a) and the modulator part (c) is low.
In the semiconductor laser device shown in FIG. 27, the cladding layer 350 above the ridge 140 is employed to make an ohmic contact with the electrode 700. However, since the electrode 700 is disposed in a wide range, current flows between the laser part 1010 and the modulator part 1020 through the electrode 700, thereby reducing the isolation resistance between the laser part 1010 and the modulator part 1020. This results in wavelength variation, and further degradation of the transmitted waveform during long-distance transmission in an optical fiber.